Processes for preparing ABS polyblends are known in the art. Such processes include the mass, mass/suspension and emulsion polymerization of prepolymerized rubber with monomer formulations of styrene and acrylonitrile monomers. Emulsion polymerization is often preferred because larger percentages of rubber may be grafted and incorporated in the polyblend. U.S. Pat. No. 3,509,237 disclosed mass/suspension and emulsion polymerization processes for preparing ABS polyblends. U.S. Pat. No. 3,957,912 discloses a mass process for preparing ABS polyblends based on emulsion grafted rubbers.
The emulsion polymerization of a prepolymerized rubber with said monomer formulation in aqueous emulsion provides a latex comprising grafted rubber particles, said particles having an average diameter in the range of 0.005 to 2.0 microns. As those skilled in the art realize, such latices can contain a certain amount of coagulum which can be ungrafted rubber that agglomerates to large particles or polymeric scale from the reactor that can be deposited at high shear areas in the reactor and flake off into the latex. Such materials, called coagulum, are generally removed from the latex by some form of low shear filtration before the latex is processed further to form a polyblend. If such coagulum is allowed to remain in the latex and is carried onto the final polyblend, such coagulum will cause hard spots in the final ABS formed parts.
The filtering of such latices through tightly woven cloth filters will remove such coagulum, however, such filtering means blind quickly and are not practical commercially. Filter beds of sand have been used, however, any filter-bed sand contamination coming through into the final product is unacceptable. Fine screens have been used but high shear through such screens, which blind readily, produces additional coagulum.
It is the objective of the present invention to provide a process that will prepare said grafted rubber latices free of such coagulum by passing said latices through a novel porous filter bed means comprising agglomerated particles of said latex as a filter bed medium. It has been found that the agglomerated particles form a crumb with a particle size distribution that in bed form has a large number of pores and interstices and provides a very efficient filtering medium for removing coagulum and passing the aqueous latex containing the small grafted rubber particles. The novel filtering medium has been found to have the additional advantage that the filter medium does not contaminate the product if it passes through into the filtered latex as it is indistinguishable from the final product having the same composition as the grafted rubber particles.